Compensated, controlled-feedback, amplitude modulated oscillator



P. WEATHERS COMPENSATED CONTROLLED-FEEDBACK AMPLITUDE MODULATEDOSCILLATOR May 10, 1949.

Filed Aug. 22, 1947 w w W M a w w W W 0 w my m m w 4 mm fl m 2 y W am 2M u #7 T a 1 l4 .1 M Q l I m. a m w w 4 F 1, m J a .4. a .w m

i .W mw mm .0 I 11 M w M Y I. Mr M .Ufl ZlfiM a m a. 1, WE", J mI J! M(I .1 a I d, mm W M J a J m g n 2 m M m m v a a v F a WWW A fi w k 0.WWW WR km 4 IN VEN TOR. PAUL .Wsxraans Patented May 10, 1949 UNITEaeca'sos rric MPENSA'EED, CONTRULLED-FEEDACK,

v.ilflhll'EE MODULA'EED @SCELLATOR llaui Weatherafiaddon Heights, N. 3.,assignor to Herbert K. Neuber, Philadelphia, Pa.

Application August 22, 1947, Serial No. 710,069 1 (Elaine. (Cl. 332-48)The present invention relates to modulated oscillators of the electronictube type as shown. described and claimed, for example, in my copendingapplications Serial No. 636,702, filed December 22, 1945, forOscillators, now Patent No. 2,436,129, granted Feb. 17, 1948 and SerialNo. 715,377, filed December 11, 19%, for Oscillator, now Patent No.2,436,129, granted Feb. 17, 15348.

In the first oscillator system referred to above, a self-excitedelectronic tube oscillator is provided with means for effecting avariable control of oscillation strength or modulation efiect thereon,and a resultant amplified modulation signal output. Modulation isapplied to the oscillator in response to extremely small changes siredmodulation efi'ects nd without appreciably in capacity, inductance orresistance in a control variation.

In the second of the above applications a selfcxcited, electronic-tubeoscillator system is prosystems referred to.

vided with a modulation control circuit for varystrength of theoscillation without appreciably varying the frequency of theoscillations produced, whereby a greatly increased high fidelitymodulation signal output is attained.

In each of these oscillator systems differentially variable capacity,inductance or resistance de vices provide the modulation source. Inpreferred embodiments of these oscillator systems a phonograph pickupdevice as the modulation source and in the form of difierentiallyvariable capacity actuated by movement of the pickup stylus, serves tomodulate the oscillator by the difierential control of feed-back energyfrom the anode circuit to the grid circuit.

In any case, the modulation source may com prise a small, light-weightpush-pull variable capacitor or any similarly variable impedance, andthe oscillator and associated circuits may be coupled thereto through atransmission line of appreciable length, without introducing undelimiting the frequency range of response of the impedance element, orthe amplitude of the con trolling effect upon the oscillator and thefidelity of the resulting signal output therefrom.

In the reproduction of records by means of a pickup device of thecharacter referred to, tonearm resonance or other undesirable frequencycharacteristic may be introduced into the modulation efiect upon theoscillator system, thereby causing distortion in the reproduction or alimitation of the efiective overall gain of the oscillator in thereproduction of signals from the record at the stylus.

It is, therefore, a primary object of the present invention to providean improved modulated oscillator circuit which effectively operates tocorrect for any undesired frequency characteristic which may be appliedto the oscillator as a result of modulation.

The invention is not limited to the oscillator However it isparticularly effective in and adapted to a system as shown, describedand claimed in the second of the applications hereinbefore referred toand will be shown and described in connection with a modulatedoscillator system of that type.

It is, therefore, an object of the invention to provide an improvedmodulated oscillator of the self-excited electronlc-tube type havingdifierential modulation iced-back control for varying the magnitude orstrength of the oscillations at the modulation frequencyby varying theinput capacity of the oscillator, and controlling the voltage gain ofthe oscillator in .counter-phase to the modulation by controllednegative or inverse modulation frequency feed-back, whereby a desiredmodulation frequency correction may be efiected to eliminate anundesired modulation characteristic.

The invention, furthermore, is not limited to the reproduction ofphonograph records in connection with the modulation of an oscillatorbut may be applied to any modulated oscillator system, the modulationcharacteristic of which requires correction in one or more portions ofits frequency range. I

It is, therefore, a further object of the invention to provide amodulated oscillator systemwhich may be efiective in any portion of themodulation frequency range and which maybe embodied in or form part oi.the oscillator circuit, and also which may take advantage of the gain ofthe oscillator tube to amplify the corrective effect upon the modulationcharacteristic.

It is a still further object of the invention to provide a self-excited,modulated oscillator system having an improved modulation controlcircuit which will permit the oscillator signal output to be adjusted tooptimum value in the presence of any undesired modulation characteristicwithout introducing cross-modulation, through correction of thepercentage of modulation in the response range in which the undesiredcharacteristic occurs.

For example, in the reproduction of phonograph records tone armresonance may introduce a rapid rise in the percentage of modulation atlow frequencies and this may in turn produce such resultant highmodulation percentage that cross-modulation of the high frequencies mayoccur.

It is, therefore, an object of the invention also to provide an improvedmodulation control system for an electronic-tube modulated oscillatorwhich introduces degeneration into the oscillator system at apredetermined frequency or frequency range whereby the percentagemodulation may be corrected in that range and in which the degree ofdegeneration may be established at any desired predetermined value andthe resultant frequency compensation may be adjusted so that themodulation percentage in the range referred to is controlled to a degreewhereby it becomes possible to increase the overall gain or sensitivityof the oscillator without appreciable cross-modulation efiect resultingfrom undesired modulation effects from the modulation source or theassociated circuits.

The invention will, however, be better understood from the followingdescription, when con-' sidered in connection with the accompanyingdrawing, and its scope will be pointed out in the appended claims.

In the drawing:

Figure 1 is a schematic circuit diagram of an electronic-tube modulatedoscillator system embodying the invention in a present preferred form;

Figure 2 is a schematic representation of the mechanical elements of aphonograph record reproducing system with which the circuit of Figure 1is adapted for use;

Figure 3 is a graph showing curves illustrating certain operatingcharacteristics of the circuit of Figure 1 in accordance with theinvention, and

Figure 4 is a second schematic circuit diagram showing a modification oithe circuit of Figure l as a further embodiment of the invention.

Referring to Figure 1, an electronic-tube oscillator 5 is arranged forgenerating self-oscillations and for receiving modulation from a source6 which itdelivers at a highly amplified value to output terminals I and8, for example as described in my aforesaid application.

In accordance with the invention, the oscillator grid 9 is connectedthrough a coupling or grid capacitor ill with the high potentialterminal I l of a grid circuit l2 in which is connected a variabletuning inductance I3, between the terminal and a ground connection leadit for the oscillator system. The cathode l5 of the oscillator tube 5 isalso connected to the ground lead [4 for the oscillator system. Thecathode l5 of the oscillator tube 5 is also connected to the ground leadi4 through a feed-back modulation frequency correction net-workindicated at I6 and comprising in the present example a feed-backimpedance H in the form of a resistor, which may be variable asindicated, and a shunt capacitor l8 therefor:

A grid resistor I9 is connected directly between the grid and thecathode as shown, forming a D.-C. biasing path. The resistor l9 may havea value of several megohms, such as ten megohms for example.

The output anode of the oscillator tube 5 is connected through avariable tuning inductance 2| in series with an output couplingimpedance or resistor 22 to a suitable source of positive anodepotential indicated by the lead 23. The lead 23 is provided with aby-pass capacitor 24, and the anode circuit lead 25 between theinductance 2| and the resistor 22 is by-passed to ground through aby-pass capacitor 26 for the oscillator frequency.

It will be noted that the output terminal 8 is also connected to theground lead It and that the output terminal I is connected through acoupling capacitor 21 and filter resistor 28 in series, to the anodecircuit lead 25.

A variable tuning core 29 is provided for the anode circuit inductance2| as well as a shunt tuning capacity therefor indicated at 30. Thelatter comprises the inter-electrodal capacities of the tube and thereflected reactance through the inductance 2| from a modulator circuitcomprising a feedback inductance 3| and a transmission line having leads32 connected at one end of the outer terminals 33 and 34 of theinductance 3| and having the opposite end connected with a pair ofspaced capacitor plates 35 and 36 in the modulation source 6. The leads32 may be extended over a considerable distance to the modulation unit6, and are suitably shielded as indicated at 31, the shielding elementbeing connected to ground through a lead 38. The stray capacity betweenthe leads 32 and the shield 31 are indicated by the capacities 40 and Mwhile the stray capacity between the leads is indicated by the capacity42.

The feed-back inductance 3| is provided with a center tap 43 which isconnected conductively through a lead Mi with the hi h potentialterminal ll of the grid circuit. The latter circuit is variably tuned bymeans of a movable tuning core 45 associated with the grid inductanceHi, and a variable shunt capacity 46 representing the input capacity ofthe tube and comprising mainly the reflected grid-to-anode capacity ofthe tube, together with the stray capacity and the grid-tocathodecapacity of the tube. This input capacity varies with the gain of thetube which is controlled by modulation to effect a variation of thestrength or magnitude of the oscillations and the modulation output fromthe oscillator.

With the arrangement shown, the tube 5 will operate in aself-oscillating condition because of energy feed-back across theinductance 2| through grid-to-place capacity from the anode circuit tothe grid circuit l2. When the tube operates, the anode current flowthrough the resistor 22 decreases because of an increase in negativebias on the oscillator grid 9 established by the grid current flowthrough the grid resistor I 9, until a steady state of oscillation isestablished. The oscillator is thus self-excited and tends to oscillateat a fixed frequency determined largely by the circuit constants in theanode circuit.

Modulation of the oscillator is provided by feed back from the anodecircuit to the grid circuit through externalmeans comprising thefeedback circuit inductance 3| which is coupled to both the grid circuitand the anode circuit and the balanced transmission line 32 and themoduaeeaeos lation source connected therewith. The tap 33 is at theinductive center of the feed-back inductance 3i which is closely coupledinductively with the anode circuit inductance 2!, and is directlycoupled to the high potential grid circuit terminal it through the lead36, whereby icedback current from the anode circuit may flowtherethrough to the grid circuit.

In the modulator unit it a movable electrode or capacitor plate 38 islocated between the fixed plates 35 and 3% and pivoted as indicated at39, for oscillation as indicated, between the fixed plates, to vary theair gaps on 'eachsicle thereof inversely, thereby correspondinglyvarying the capacity of the fixed electrodes with respect thereto andthe electronic impedance of the circuits connected with the plates 35and 236 when as in the present example, the capacitor plate at isconnected to ground M. Through the connections 32 between the plates 35and 36 and the terminals 33 and 3d of the feed-back inductance 3!, theelectrical impedance to ground from'the center tap it through each halfof the inductance 3! is correspondingly inversely varied in response tomovement of a stylus or other movable actuating element 50 for the plate$8.

With this arrangement, the feed-back induct ance 3! is provided with twonormally balanced feed-back paths to ground or cathode which producesbalanced positive and negative feed-back through feed-back inductancewhich may be varied differentially to modulate the oscillator. This isby reason of the fact that as the flow of energy or current with-respect.to the tap 53 through one winding portion of the inductance 3iincreases, the flow through the other winding portion decreasesproportionately, thereby effecting a push-pull action to increase anddecrease the strength of oscillations of the oscillator 5. The resultinganode current variations represent the amplified signal from themodulation source which is derived at the modulation frequency acrossthe impedance element 22 from the terminals 7 and B.

The foregoing modulation system may provide any suitable means at themodulation source in connection with the feed-back inductance 39 forconveying feed-back current or energy from the anode circuit throughsaid inductance difi'erentially, to provide differential in-phase andcounter-phase inductive feed-back or energy to the grid circuit therebyto vary the amplitude or strength of oscillations of the oscillator.

If the modulation source ii is subjected to the undesired modulationaction in operation, as for example, when the operating element c isarranged as the stylus element in the end of a phonograph tone-arm 52 asshown in Figure 2, to which attention is nowdirected, it may have anon-uniform frequency characteristic in the low Figure 2, and areconnected with the terminals 33 and 38 as shown in Figure 1.

When the stylus 50 is actuated by constant amplitude recording, for'example, as in present phonograph records, below 500 cycles and above2000 cycles, tone arm resonance may cause the percentage of modulationto exceed a desirable limit and result in a high percentage modulationWithin the range of frequencies in'which the tone arm resonance occurs.This same result or eiicct may occur in other forms of reproducingequipment, that is, with other forms of modulating devices at themodulation source 5 and in turn may require correction in the particularfrequency range in which the over-modulation or undesired frequencycharacteristic occurs. The modulation system of the present inventionmay be adapted to meet the requirements for modulation frequencycorrection in any of the above cases, as

will be seen from a consideration of this present per cent modulation asshown. This is a normal response curve for a phonograph pickuparrangement as shown in Figure 2 when connected with a modulatedoscillator as in Figure 1, without provision of frequency response ormodulation control means in accordance with the invention.

The rapid rise at the lower frequency end of i the range, particularlybelow 50 cycles, is the reexample, may excite the tone arm stylus toproduce suchhigh modulation percentages as in dicated by'the portion ofthe curve ti, that cross-modulation of the high frequency portions ofthe reproduced signal by the low frequency modulation referred to mayoccur.

In accordance with the invention, effective degeneration or inversemodulation frequency feedback is provided at low frequencies, or in adesired frequency range, so that the percentage of modulation may bereduced to a value such that audio frequency range when the tone armresonance becomes effective, tending to produce dis= no appreciablecross-modulation results. In the present example, the percentage ofmodulation is reduced to a value at the tone arm resonant i'requency'such that the cross-modulation is substantially zero.

The curve 62 shows the resulting output which may be effected by thismeans. It will be noted that the lower end of the curve 82, as indicatedat 63, provides selectively low percentage modulation rise which iswithin a desired limit for preventing cross-modulation of the higherire-. quencies in the signal output from the system. It will also benoted that the curve $2 is at a generally higher level, of the order oftwo to one, than that of the curve 52', which indicates the t theoverall signal output may be increased greatly without attendantundesired over-modulation at the low frequency end of the responserange, whereby .the system may efiectively be used in connection withthe reproduction of phonograph records with a normal tone armarrangement as shown, for example, in Figure 2.

Referring again to the circuit diagram of Figure 1 more particularly,the oscillator 5 is maintained in a steady state of oscillation by thenormal feed-back through the grid and anode capacity as hereinbeforereferred to, and the modulation from the source 6 is applied thereto byinverse or differential variation of the external feed-back pathprovided between the anode and grid circuits through the feed-backinductance 3!, the pickup circuit 32 and the differentially variablecapacity provided between the movable pickup element 48 and the fixedelements 35 and 36.

The frequency corrective network utilizes a third feed-back path andmodulation frequency feed-back as distinguished from the oscillatorfrequency feed-back for the modulation between -the anode and the gridcircuit. The negative or inverse modulation frequency feed-back isapplied to the grid by the impedance or resistor 11 in the cathodecircuit which is given a desired frequency response characteristic bymeans of the shunt capacitor H3 or other suitable means as willhereinafter be described.

It will be seen that the resistor I! is located at a point in theoscillator circuit whereby it couples the grid and anode circuits and isin effect a portion of the output impedance 22 located in the anodecircuit, whereby anode current variations resulting from modulationappear across the terminals of the impedance or resistor IT. Thisvariation is applied to the grid circuit through the inductance l3, andbecause of the polarity of the connections, the resulting potentialvariation at H is in counter-phase or degenerative with respect to theinput signal or modulation appear: ing on the grid 9. This circuitarrangement operates to reduce the percentage modulation due to themodulation voltage feed-back being in such phase that the voltage gainof the oscillator is varied in counter-phase to the modulation envelope,as will be described in connection with the operation of the system.

To provide for frequency compensation or correction of the modulationeffect in the present example in the tone arm resonant range, the shuntcapacitor I8 is of such value that it forms an effective by-pass for theresistor I! at all of the higher frequencies, for example above 500cycles, Since the grid resistor IQ for the oscillator is returneddirectly to the cathode IS, the 'bias on the grid 19 is not changed bythe variation in modulation potential at the resistor i1. However, whenthe anode current changes, the change also occurs across the resistor I!at frequencies below that frequency at which the impedance of thecapacitor I8 becomes too large appreciably to shunt the resistor H, inthis case, below 500 cycles. The resistance value of the latter resistormay be made a predetermined percentage of the resistance value of theresistor 22 so that a predetermined degree of degeneration or inversemodulation frequency feed-back may be obtained, and the degree offrequency compensation may further be adjusted by variation of theresistor [1. By this arrangement the low frequency modulation percentagemay be reduced to a point where it becomes possible to increase theoverall sensitivity of the modulated oscillator system as shown inFigure 1, without the possibility of cross-modulation.

Suitable amplification of the reproduced signal from the modulationsource 8 may be introduced between the oscillator tube 5 and the outputterminals 1 and 8, and the inverse modulation frequency feed-backcorrection network l6 may be provided with additional degenerative orinverse feed-back from one or more amplifier stages connected with theoscillator, as shown, for example, in Figure 4, to which attention isnow directed, and in which the same reference numerals as used inconnection with Figure 1 are applied to like circuits and circuitelements.

In the circuit of Figure 4, the modulation source and oscillator circuitper se is the same as in Figure 1 and functions in the same manner toproduce self-oscillations and a modulation signal output at theterminals 1 and 8 from the source 6. The feed-back modulation frequencycorrection network it in circuit with the cathode l5 comprises aresistor 65 connected between cathode and the ground lead II and shuntedby a resistor or other suitable impedance 66 in series with a capacitor61.

The oscillator output signal through the output capacitor 21 is appliedto the grid 68 of an amplifier tube 69 havin cathode 10 connectedthrough a self-bias resistor H to the ground lead 84. The self-biasresistor H is provided with the usual by-pass capacitor 12 to ground andthe output anode 'I3,is connected across an output.

impedance or resistor 14 to the output terminal 1 through a couplingcapacitor 15. The output terminal 8 is connected to ground as inFigure 1. Anode potential is applied to the anode 13 through a positiveanode potential supply lead 16 which is by-passed to ground by a by-passcapacitor indicated at l1. Bias is applied to the grid through aresistor 18.

Additional inverse feed-back is applied across the resistor 65 in thecathode circuit of the oscillator 5 through 'an inverse or negativemodulation frequency feed-back connection lead connected between thecathode end of the resistor 65 at a terminal 8| and a tap 82 on theoutput impedance 16, through a series coupling capacitor 83 in the lead80. With this arrangement a portion of the signal appearing across theoutput impedance I4 is applied substantially in phase with the anodepotential modulation frequency variations in the resistor 65 resultingfrom the modulation, and tends to enhance the inverse feed-back ordegenerative effect upon the modulation si nals on the grid circuit,thereby further to control the output response of the modulatedoscillator in a desired frequency range.

In the present example, a modulation control or corrective circuit isprovided which is effective to suppress undesired frequency response inthe system in a band of frequencies, for example, between 500 cycles and2000 cycles or in any similar frequency pass band, being a modificationof the corrective network arrangement of Figure 1 for other purposes. Asin the previously described circuit, the modulation frequencycharacteristic is corrected by inverse feed-back in a circuit externalto the self-excitation and modulation circui s.

In the present example, resistor' 66 and the capacitor 61 form a shuntpath for the modulation frequency feed-back impedance 65 which has acontrolled frequency characteristic such that below a certain frequency,such as 2000 cycles for example, the impedance of the path formed bysaid resistor and capacitor becomes increasingly effective to permit theinverse modulation frequency feed-back to be applied to the grid cir-fcult, thereby suppressing the signal output in a range below thatfrequency.

The capacitor 83, however, is of such value that at the lower limit ofthesuppression range desired, such as at 500 cycles in the presentexample, it becomes increasingly eflective in impedance value to preventthe flow of feed-back current through the circuit 80, thereby preventingreduction of the gain of the system below that frequency and cutting offthe effectiveness of the by-pass arrangement 86 and 67 for theadditional feed-back from the amplifier. It is obvious that othersuitable bandpass control networks may be devised for providingfrequency characteristic correction in any desired band or range as maybe required.

In any case, from the foregoing consideration of the circuits of Figure1 and Figure 4, it will be seen that the feed-back modulation fre quencycorrection network is placed in the oathode circuit of the oscillator sothat it eifectively couples the grid and anode circuits substantiallyindependently of the difierentially controlled modulation circuit.Furthermore, with this arrangement, the inverse feed-back is amplifiedby the gain of the oscillator tube itself and therefore the correctioneiiect is greatly enhanced over that which would be possible by use ofordinary series circuit limiting or other known means.

The operation of the system shown in Figure 1 may briefly be consideredas follows:

Assumin the oscillator to be energized, feedback of energy from theanode circuit through the normal grid-to-anode capacity path maintainsthe oscillator in a steady state of oscillation at a frequencydetermined by the anode and grid circuit constants. In the presentexample this may be assumed to be of the order of 1445 kc. The averageanode current flowing through the output coupling impedance 22 and thefeed-back correction impedance il in a cathode circuit, assumes a normaland constant value, and with substantially no variation in average anodecurrent resulting from no excitation a variation of the modulationsource capacitor 35--36t8, the modulation signal output at theterminals'i and 3 is zero. Energy from the anode circuit at the oscillatorfrequency is prevented from appearing at the output terminals by reasonof the bypass capacitor 2e and the filter 28 connected with the terminal3.

Modulation feed-back current at the frequency of the tuned anodecircuit, that is, at the normal oscillator frequency, flows by inductivecoupling through the feed-back inductance 3i substantially equally oneither side of the intermediate tap it. Modulation feed-back currentthrough the inductance 3i flows in a path which may be traced from thecenter terminal d3 through the connection lead its? to the grid circuitinductance l3, thence through the ground connection it and the movableelectrode 58, thence through the electrodes or plates 35 and 36, thecable connections 32 back to the terminals 33 and 3d of the inductance38.

It will be seen that when the return path through the leads 3? and themodulation source 6 is balanced, that is when the impedance to ground inconnection with the terminals 33 and 3d are equal, the opposingpotentials induced in the grid circuit through the coupling with thefeed-back inductance 38 are equal and opposite in phase and cancel,whereby the steady state of oscillation is maintained without change.

Variation of the impedance element at the l control end or themodulation circuit, 32 causes a variation in the impedance betweenground and the terminals 33 and as, thereby causingan unbalance in thepotentials applied to the terminal 63 and the grid circuit connectedtherewith difi'erentially, thereby causing a flow of iii-phase orcounter-phase feed-back energy to the grid circuit, and increasing ordecreasing the strength of oscillations.

This is by reason of the fact that when the feed-back of energy throughthe feed-back inductance Si is varied, and increase in feed back fromthe anode circuit in one direction will aid and in the oppositedirection will oppose, the normal reed-back from the. plate to the grid,thereby varying the amplitude of oscillations and the signal output atthe terminals l and d in response to modulation control of the variableimpedance means at the modulation source d.

Variation in reed-back of the oscillator signal energy causes the inputcapacity as across the grid circuit to vary, thereby varying thefrequency of response of the grid circuit with respect to the fixedfrequency or the oscillator circuit. The variation of t of the gridcircuit, however, has substantially no efiect upon the fre quency of theoscillator which is maintained substantially constant by the tuned anodecircuit comprising the inductance 2t and the-reflected capacity 3E! ashereinbefore referred to. Variation of the of the grid circuit byvariation of the input capacity only varies the magnitude or strength ofthe oscillations. Therefore, in this system, modulation is effected byvarying the strength or magnitude of the oscillations by varying theinput capacity of the oscillator tube in response to differentialvariations of feed-back of energy at the oscillator frequency from theanode circuit to the grid circuit, with respect to the normal feed-backfor a steady state, self-excited condition of oscillation.

The anode current is correspondingly varied at the modulation frequencyand provides a corresponding modulation potential variation across theoutput impedance element 22 and the modulation control feed-backimpedance ll, the voltage across the latter impedance introduces inverseor degenerative modulation frequency feed-back in the grid circuit ofthe oscillator to reduce the percentage modulation applied thereto andthe resulting signal output at the terminals 7 and t3 correspondingly.

Correction of the frequency characteristic by negative or inversemodulation frequency feedback takes place in the grid circuit. whenapplied to the network 5 5 in the cathode lead, as follows: Thepercentage modulation is reduced by feedbackirom the plate-to-anodecircuit'to the grid circuit due to the fact that the modulation voltageis fed back in such phase relation that the voltage gain of the tube iscaused to vary in counter-phase to the modulation envelope. Since theeffective grid-to-anode capacity, or input capacity, appearing acrossthe grid circuit is essentially proportional to the voltage gain of theoscillator tube, fluctuations in the voltage gain of the oscillator tubeoccasioned by negative modulation feed-back limits the maximum swing orvariation of the efiective tuning capacity across the grid circuit, andtherefore reduces the percentage modulation.

The feed-back voltage across the impedance ll is modified to apply adesired correction to the modulation amplitude thereby to compensate forany undesired frequency characteri tic resulting from the modulation atthe source 8; In the present example, as hereinbefore referred to, thecapacitor It forms an effective bypass for the inverse feed-backmodulation frequency potentials above a certain low frequency range sothat the degenerative eifect of the correction network becomes effectiveonly below, for example, 500 cycles, and increasingly eflective in therange of the tone arm resonance frequency, thereby preventingover-modulation andpermitting an overall increase in the signal outputby adjustment of the system to optimum amplification, as indicatedby acomparison of the curves 6i and 62 in Figure 3.

In the circuit of Figure 4, additional gain in the modulation signaloutput'is provided at the terminals 1 and 8 by introduction of theamplifier stage associated with the amplifier tube 69, and additionalcorrective inverse feed-back is obtained from the amplifier stage outputcircuit by reason of the feed-back connection between the tap 82 and thecathode end of the oscillator cathode impedance 65. By proper relationof the impedance elements 83, 66 and 61 a band-pass effect for frequencyresponse correction may be applied. Furthermore, by making the tap 82adjustable as indicated, the degree of feed-back from the amplifierstage may be controlled and likewise the impedance 65 may be madeadjustable as shown in connection with the similar impedance IT in thecathode circuit of Figure 1.

In any case, the feed-back modulation frequency correction network isapplied to the oscillator system at the cathode circuit to provide foramplification of the correction eflect by reason of the gain in theoscillator tube itself.

The present invention is, therefore, of importance in connection withthe reproduction of phonograph records and the like, through the usualtone arm and pickup arrangement, as it will be seen that it may be madehighly effective to control the modulation within any desired frequencyrange, thereby to impart to the output signal a desired frequencycharacteristic devoid of any undesired modulation effects by reason ofoperation of the modulation source and its associated elements, such astone arm resonance, for example.

Furthermore, while the invention has been shown and described inconnection with a present preferred form of modulated oscillator, and isparticularly efiective in connection therewith, it may be applied to anymodulation control system for a self-excited electronic-tube oscillatorprovided with means for efiecting a variable control of oscillationstrength or modulation effect by inverse variation of feed-back.

What is claimed as new and useful is:

In a self-excited electronic-tube oscillator system, the combination ofmeans for tuning said system to a predetermined constant frequency ofoscillation, modulator means for difierentially varying feed-back ofenergy at the oscillator frequency to modulate said system in apredetermined modulation frequency range, means for applying thereto acontrolled negative modulation frequency feed-back for limiting theefiect of variation of said oscillator frequency feed-back to controlthe percentage modulation in a predetermined portion of said modulationfrequency range, said last named means including an oscillator cathodecircuit and an impedance network in said circuit, amplifier means forthe modulated signal output of said system having an output impedance,and a negative modulation frequency feed-back connection from said lastnamed impedance to said network for further controlling the percentagemodulation of said system in a predetermined portion of said modulationfrequency range.

PAUL WEATHERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,101,688 Rechnitzer Dec. 7, 19372,371,373 Badmaiefl' Mar. 13, 1945 2,412,023 Woll Dec. 3, 1946

